You can get a razor scraper with a long handle at one of the big box stores. 4-5" blades, buy extras!

Depending on how even the layer is, you might just put your new decoupling layer (cbu ) on top of it with new thinset. The existing stuff must be well adhered. If you wanted to use a membrane instead (like ditra), you'd probably want to get the old stuff off first. The thinset under cbu is just to fill voids and give 100% support, but the thinset under a membrane is what holds the membrane in place.

All cbu manufacturers REQUIRE thinset (that I know of anyways). The thinset can be cheap dryset mortar (i.e., unmodified), since its sole purpose is to fill voids and provide 100% support. The nails or screws are what hold it down in this case, not the thinset (although it does somewhat). Make sure to also use the proper mesh tape on the seams. that can be installed during tiling so you don't generate speed bumps. Taping cbu seams is entirely for strength, and not for appearance like it is on drywall, so no tapered seams...just embedded in the thinset while tiling is easiest. Make sure the mesh tape is spec'ed for use with thinset...that for drywall will eventually disintegrate in the alkalai environment.

If you can't put up with the increased floor height by going over it with thinset and cbu then you can try a chipping hammer. There are these flat blades that have a real sharp thin edge that are used to remove thinset and tile. It's very think like sheet metal. Works pretty good.

If you can't put up with the increased floor height by going over it with thinset and cbu then you can try a chipping hammer. There are these flat blades that have a real sharp thin edge that are used to remove thinset and tile. It's very think like sheet metal. Works pretty good.

Click to expand...

So -- I can just thinset and cbu over the exisiting thinset??
If yes -- I'd rather do that than trying to remove it.

Yes, if the thinset is firmly attached and moderately level, you can add more then attach the cbu. BUT, keep in mind the original layer may crack and because it is hard, make it really hard to get the cbu to lay flat without voids. The whole reason for the cbu is two-fold: make a compatible bonding layer for the tile and decouple with the floor beneath. The reason for the thinset is to ensure 100% support.

If the thinset is fairly smooth and level and properly bonded to the subfloor, I'd consider installing Ditra from www.schluter.com over it. This is relying on the thinset to hold it in place so there'd be no likelyhood of cracking the old, hardened thinset when you add the screws or the nails. If the existing thinset is NOT firmly bonded, though, there'd be a compromised connection holding the Ditra in place.

A rubbing stone could probably take off any high spots. It's better to remove it, though.

In the end, though, it might be less time consuming to just replace that layer of ply with new - less time, better end result; known condition.

What Jim said. Just also make sure to check your floor specs for tiling. I like the idea of true thinset, then CBU. You can even install Ditra on top of that if your structure is somewhat unknown. But ideally, you want proper plywood and joist support for a long-live tile job. As mentioned earlier, do check out johnbridge.com for more...Greg

PS Jim, about thinset underneath CBU- surprisingly, this used to not be part of the installation instructions years ago, but has obviously since changed and for good reasons.